In the production of a compound semiconductor epitaxial substrate by allowing each of compound semiconductor layers to epitaxially grow on a base substrate, there is used an epitaxial growth method (hereinafter referred to as “selective growth) that includes a selective growth step wherein a mask comprising SiO2, etc is formed on a functional layer of a compound semiconductor provided on the base substrate and a layer of compound semiconductor is grown on a portion in which the mask is not formed. A compound semiconductor epitaxial substrate having a pn junction (including a p-i-n junction) is produced by this selective growth. By providing electrodes and the like thereon and dividing the resulting devices, compound semiconductor devices such as field-effect transistors (other than schottky junction type) (FETs) and hetero bipolar transistors (HBTs) are produced.
As a base substrate to produce compound semiconductor epitaxial substrates for these compound semiconductor devices, a base substrate which comprises a compound single crystal produced by Liquid Encapsulated Czochoralski method (LEC method) is usually used (for example: Patent Document 1).
This LEC method provides a single crystal substrate in high yield and it is low cost. However, it is known that a residual strain in the produced base substrate is 1.2×10−5-1.0×10−4 probably because of a high temperature gradient during the crystal growth of a compound semiconductor (for example: Patent Documents 1 and 2). However, the LEC method is supposed to be sufficient in the production of a compound semiconductor epitaxial substrate produced by selective growth for the device of FETs (other than Schottky junction type) and HBTs and therefore, a base substrate produced by LEC method have been conventionally used in the production.
On the other hand, less lattice defect in compound semiconductor crystal is required, and as a base substrate which is used in the production of a compound semiconductor epitaxial substrate for the production of devices such as a laser diode without recourse to selective growth, further less residual strain is required. Vertical Gradient Freezing method (VGF) and Vertical Bridgman (VB) method were proposed as a production method for that base substrate, and it was known that the residual strain of a base substrate produced by these production methods were about 1×10−6 to 5×10−6, which was lower than those of base substrates produced by LEC method (see Patent Document 1). However, the base substrates produced by VGF method and VB method had not been used for production of a compound semiconductor epitaxial substrate having a pn junction by selective growth because the initial electric characteristics of devices such as FETs (other than Schottky junction type) and HBTs were not remarkably improved and the costs of these methods were higher than those of LEC method.
It is known that a compound semiconductor device produced from a compound semiconductor epitaxial substrate having a pn junction by selective growth has problems of occurring of characteristics deterioration with use for a long time, so improvement of the problems had been demanded.
Patent Document 1: JP 11-268998 A
Patent Document 2: JP 5-339100 A